The conventional electrophotographic process has an inherently lower gain than the silver halide photographic process. A low exposure in a conventional electrophotographic process results in a low amplitude differential voltage pattern on a photoconductor, and when developed with conventional toner, the resulting toned image has a low density. It has been a longstanding goal to increase the gain of the electrophotograpic process so that higher density images may be produced from low exposures. This is of particular concern in applications such as diagnostic xeroradiography, where the exposing X-rays pose a potential health threat to the patient, and the lowest exposure possible is desired.
In addition to the conventional xerographic process, there are other electrographic processes that produce weak differential patterns of voltage, charge, current, or conductivity and for which increases in gain or photographic speed are desirable. Such electrographic processes include, for example, photoelectrophoresis (see U.S. Pat. No. 4,361,636 issued Nov. 30, 1982 to Isaacson et al.), ionography (see U.S. Pat. No. 4,070,577 issued Jan. 24, 1978 to Lewis et al.), and ion projection (see U.S. Pat. No. 4,338,614 issued July 6, 1982 to Pressman et al.).
It has been proposed to increase the gain of an electrophotographic system, particularly a xeroradiographic system, by amplifying a low amplitude differential voltage image produced by a low X-ray exposure (see U.S. Pat. No. 3,981,727 issued Sept. 1, 1976 to Nelson et al.). In the method of signal amplification taught by Nelson et. al., a low amplitude differential voltage pattern (signal) is developed with an opaque toner. The charged photoconductor, with the image in place, is uniformly illuminated to reexpose the photoconductor using the toned image as a mask. The reexposed image is then further developed by applying additional toner to increase the density range of the image.
Our theoretical studies of the type of signal amplification disclosed by Nelson et al. show that this approach is inherently limited to producing threefold or fourfold increases in gain relative to conventional electrophotography. This conclusion is due mainly to the fact that with low initial exposure the toned image produces a low optical density mask that is somewhat transparent even in the highest density areas. As a result, the photographic speed of the Nelson et al. process is not high. Further increases in the gain of electrophotographic systems are desirable.
It is an object of the present invention to provide a method of amplifying an image in an electrophotographic process and to produce further increases in gain and higher photographic speed thereby. It is a further object of the invention to provide means of amplifying low amplitude voltage, charge, current or conductivity patterns produced by other electrographic methods, such as ionography, stylus recording, ion projection, and photoelectrophoresis.